(1) Field of the Invention
The invention relates to a sealing device for a bearing in a machine tool, in particular, to a device for preventing cutting fluid from entering a bearing on a spindle in a machine tool.
(2) Description of the Related Art
A machine tool, such as a CNC drilling machine, is often installed in an environment where a large quantity of a mist or small droplets of cutting fluid are generated by a cutting process and float in the air. This occurs in particular, where the cutting process is performed at high speed. A cutting process at a high speed requires even more cutting fluid than usual, and this results in more mist or droplets floating in the air around the machine tool. If cutting fluid, even in the form of mist, enters a bearing rotatably supporting a spindle of the machine tool, the cutting fluid can dilute the grease in the bearing and flows out therefrom. Rust can then appear in the bearing.
In the prior art, the bearings on the spindle in a machine tool are sealed, to prevent cutting fluid from entering the bearings, by an oil seal or a mechanical seal, in order to keep the bearing clean.
However, the spindle of the machine tool rotates at a high speed and generates much heat, whereby the air around the bearing is heated. The heated air cools when the spindle is stopped. Thus, the air around the bearing expands and contracts at each change in the air temperature. In order to compensate for the expansion and contraction of the air around the bearing, the machine tool is provided with a labyrinth seal around the bearing. A labyrinth seal is a clearance or non-contact sealing means having a stationary member and a rotating member. In a labyrinth seal, which is generally used for sealing the bearing in the machine tool, the stationary and rotating members have surfaces which face to each other and are positioned perpendicular to the rotating axis. The members include a plurality of concentric circular channels and lands alternatively provided on the respective surfaces. The stationary and rotating members are combined so that the circular channels and lands on one of the members match with the lands and channels on the other member, and a clearance, which allows air to pass through the labyrinth seal, is made between the members. When the rotating member rotates at a high speed, the air within the clearance flows with the rotating member due to the viscosity. Therefore, a centrifugal force, which acts on the air flow, moves the air in the radially outward direction. Thus, the labyrinth seal, during rotation at a high speed, prevents the mist of cutting fluid from entering the bearing.
The labyrinth seal, however, does not rotate when the spindle is stopped. Thus, the labyrinth seal does not function when the spindle is stopped. While the spindle is rotating, a vacuum appeares inside the labyrinth seal. When the spindle stops, air moves through the seal to release the vacuum. A mist of cutting fluid can be drawn through the seal by the flow of air.
In addition to the above, when the air around the bearing cools, more air is drawn to the bearing due to the contraction of the air around the bearing. A mist of cutting fluid can enter the bearing, through the labyrinth seal, with the air flow. In particular, a machine tool, such as a CNC drilling machine, comprises a vertical spindle and a spindle head supporting the spindle, and a mist of cutting fluid often forms condense on the top surface of the spindle head. When the labyrinth seal is stopped, the standing cutting fluid on the top surface of the spindle head may be drawn to the bearing, through the clearance in the labyrinth seal, as a flood of cutting fluid.
In the prior art, there is a device for preventing cutting fluid from entering the bearings by supplying pressurized air into the bearing.
When the pressurized air is supplied into the bearing, the grease can be forced out from the bearing by the air pressure. Moreover, a further problem is that the quality of the air supplied in the bearing must be tightly controlled by completely removing moisture and dust therefrom.